Due to rapidly rising power costs, there has been a growing demand for energy-efficient motors. Initially, the major emphasis has been directed toward improved efficiency at the expense of other valuable operating features. This is because present art motor design involves several trade-offs. Usually smaller cooling fans and bearings are used to reduced no-load or friction and windage losses. As a result, motor life is decreased because of impaired ventilation, where the motor runs hotter and this, coupled with the use of smaller bearings, adversely effects both insulation and bearing life. This is of particular importance since bearings and insulation are two major elements that effect motor life and maintenance history. Also, core or iron losses are reduced by increasing the overall length of the stator stack so that the core material can operate below saturation. Generally speaking, core material with a high permeability is used to improve or increase the magnetic flux level. This reduces core loss but, as a trade-off, requires greater magnetizing curents and therefore lowers the power factor of the machine. Since power factor is reduced, additional power factor correction capacitors are needed only adding to the overall cost of installation, operation and maintenance. Another area of improvement in present art energy-efficient motors is that of reducing branch circuit losses. This is accomplished by increasing the amount of copper used, and thereby lowering branch circuit resistance. The trade-off to increased copper fill is increased in-rush or starting current.
Manufacturers of motors and generators thus find that they must enhance or improve their product's efficiency and still provide equipment that satisfies a variety of conditions while incorporating a host of desirable operating features. Many of these features are difficult to achieve in current prior art devices such as induction motors and some features are even contradictory to one another. Examples of this would be providing a machine with both high power factor and high efficiency. In the case of medium- and high-voltage machines, the manufacturer must use more insulation to deal with the higher voltage thus forcing a reduction in the amount of copper or conductive material below that which would ensure optimum efficiency. Countless other problems are encountered in the design of induction machines, such as high in-rush or start-up currents, stray-load losses, skin and proximity losses, harmonic losses and, in the case of iron-core machines, hysteresis and eddy current losses. As both the size and operating voltage of these machines increase, many of the above problems are accentuated. Eliminating or reducing the impact of many of these inherent conditions can be difficult, costly and impractical.
In general, prior art induction machines are provided with single primary branch windings which are sized to safely handle the vector sum of the real and reactive currents required during the operation of such machines. Since these primary windings are all of the same construction and structure, they are incompatible with more than one winding type or technique. Thus, the entire winding structure in prior art machines must be designed to not only withstand the applied voltage stresses, but also to withstand the mechanical stresses caused by start-up and loading. Hence, serious problems are encountered using the same windings, especially in the case of the medium- and high-voltage induction machines, such as polyphase motors.
In medium- and high-voltage machines, the winding coils use unnecessary space for insulation that, given an alternate winding method, could be filled with conductive material such as copper to thus improve the overall operating efficiency of the machine. In one type of induction machine, such as a polyphase induction dynamoelectric device, the coils or winding structures are generally made up of large conductors having square or rectangular cross-sections which help make the coils inflexible or unyielding to movement or mechanical stress. Coils of this nature are commonly referred to as form coils. The large conductors found in form coils exhibit an undesirable characteristic called "skin effect," which is the tendency of alternating currents to flow near the surface of a conductor thus restricting the current to a small part of the total sectional area of the conductor and producing the effect of increasing the resistance.
This skin effect loss is an appreciable loss in large, high-voltage induction machines. When a true resistance value is obtained in an alternating current (AC) circuit (rather than the total impedance of the circuit), it can be seen that the effective resistance of the AC circuit is appreciably higher than the plain ohmic resistance of the same circuit when direct current flows through it. Resistance is therefore a property in an electric circuit that accounts for dissipation of electric energy. Skin effect is one of the ways that energy is dissipated in an alternating current circuit. It is not present in a direct current (dc) circuit.
By viewing the magnetic field as collapsing lines of force around an electric conductor in an AC circuit, it is observed that there are more changes in flux linkage at the center of the conductor than at its surface. This produces greater opposition to the flow of ac at the center of a conductor than at its surface. Therefore, since less current flows in the center of a conductor in the presence of ac, the effective cross-sectional area of the conductor is greatly reduced and thus the effective resistance is greater than the ohmic resistance when measured under dc.
There is thus a need for an improved induction machine in which, among other things, skin effect is reduced, and copper loss is reduced. The present invention is directed toward filling that need.